Wireless, radio frequency (RF) communication systems enable people to communicate with one another over long distances without having to access landline-connected devices such as conventional telephones. In a typical cellular telecommunications network (e.g., mobile phone network), an area of land covered by the network is geographically divided into a number of cells or sectors, which are typically contiguous and which together define the coverage area of the network. Each cell is served by a base station, which includes one or more fixed/stationary transceivers and antennae for wireless communications with a set of distributed wireless units (e.g., mobile phones) that provide service to the network's users. The base stations are in turn connected (either wirelessly or through land lines) to a mobile switching center (“MSC”) and/or radio network controller (“RNC”), which serve a particular number of base stations depending on network capacity and configuration. The MSC and RNC act as the interface between the wireless/radio end of the network and a public switched telephone network or other network(s) such as the Internet, including performing the signaling functions necessary to establish calls or other data transfer to and from the wireless units.
Various methods exist for conducting wireless communications between the base stations and wireless units. Examples include CDMA (code division multiple access), TDMA (time division multiple access), and OFDM (orthogonal frequency-division multiplexing). CDMA, widely implemented in wireless networks in the United States, is a spread-spectrum multiplexing scheme wherein transmissions from wireless units to base stations are across a single frequency bandwidth known as the reverse link. Generally, each wireless unit is allocated the entire bandwidth all of the time, with the signals from individual wireless units being differentiated from one another using an encoding scheme. Transmissions from base stations to wireless units are across a similar frequency bandwidth known as the forward link. In TDMA-based systems, which are widely used in Europe and elsewhere, frequency channels are divided into time slots for sharing among a plurality of users, e.g., the information for each user occupies a separate time slot of the frequency channel. In OFDM, the available RF bandwidth is divided into several sub-channels. The bit stream to be transmitted is split into a plurality of parallel, low-rate bit streams. Each bit stream is transmitted over one of the sub-channels by modulating a sub-carrier using a standard modulation scheme. The sub-carrier frequencies are chosen so that the modulated data streams are orthogonal to one other, meaning that interference between the sub-channels is eliminated.
Many wireless networks have a government-assigned frequency spectrum for supporting communications between the end users' wireless units and the network's base stations. Because of this limited bandwidth, and because the demand on this bandwidth increases as the number of wireless users increases, it is desirable in a wireless system to use the available frequency spectrum in an efficient manner. In particular, given a set bandwidth, greater efficiency generally corresponds to an increased number of users and/or data throughput.
To better utilize the frequency spectrum in a wireless network, service providers have begun to implement multiple-input multiple-output (“MIMO”)-based RF transmission systems. In MIMO systems, the transmitter (e.g., base station) is provided with multiple antennas capable of transmitting spatially independent signals, while the receiver (e.g., wireless unit) is equipped with multiple receive antennas. “MIMO” also encompasses systems where transmissions from a number of base stations or other transmitters are coordinated for mitigating against inter-cell interference. In both cases, by using a sophisticated signal-processing scheme to control transmissions it is possible to achieve significant increases in throughput and range without an increase in bandwidth or overall transmit power expenditure. In general, MIMO technology increases the spectral efficiency (e.g., measured as the number of information bits transmittable per second of time and per hertz of bandwidth) of a wireless communication system by exploiting the space domain, because the multiple transmission sources are physically separated in space.
MIMO systems have the potential to achieve high throughputs in wireless systems. When channel state information (CSI) is available at the transmitter, the base station can transmit to multiple users simultaneously to achieve a higher rate. For efficiently transmitting data to multiple users at the same time, the “dirty paper coding” (DPC) transmission technique has been shown to achieve the sum-rate capacity (i.e., maximum throughput) of the multiple-antenna broadcast channel. DPC is a signal processing and pre-coding scheme that allows a transmitter to send information to multiple users so that many of the users see no interference from other users, in a lossless manner (e.g., without incurring any power increase or rate loss). However, DPC is difficult to implement in practical systems due to the high computational burden of successive encodings and decodings, especially when the number of users is large.